Cardiac pacemaker lead systems fulfill two functions. The first function is to provide an electrical conduit by which a pacemaker output pulse is delivered to stimulate the local tissue adjacent to the distal tip of the lead. The second function is to sense local, intrinsic cardiac electrical activity that takes place adjacent to the distal tip of the lead.
One of the problems with such pacing and sensing lead systems is their inability to suppress or attenuate the voltage levels of far-field electrical signals. These signals are generated by depolarizations of body tissue in areas remote from the local sensing site and are manifested as propagated voltage potential wavefronts carried to and incident upon the local sensing site. A far-field signal may comprise an intrinsic or paced signal originating from a chamber of the heart other the one in which the lead electrodes are located. The sensing electrode(s) detect or sense the voltages of these far-field signals and interpret them as depolarization events taking place in the local tissue when such polarizations are above the threshold sensing voltage of the system. When far-field signal voltages greater than the threshold voltage are applied to the sensing circuitry of the pulse generator or pacemaker, activation of certain pacing schemes or therapies can be erroneously triggered.
With the development of universal stimulation/sensing systems, that is, three and four chamber combination pacemaker-cardioverter-defibrillators, accurate sensing of cardiac signals has become even more critical, and the management, suppression and/or elimination of far-field signals is vitally important to allow appropriate device algorithms to function without being confused by the undesirable far-field signals.
For a lead electrode implanted in the right atrium, the right ventricular R-wave comprises a far-field signal whose amplitude can easily swamp the smaller P-wave signal sought to be sensed. Thus, the discrimination of P-waves from the higher energy QRS complexes and particularly the R-wave spikes continues to present a formidable challenge.
Approaches to the problem of far-field signal sensing include configuring the circuitry of the pacemaker to attenuate far-field signals, and introducing a blanking period long enough to prevent the sensing of unwanted signals. These solutions are described in U.S. Pat. No. 4,513,752 assigned to the owner of the present invention.
The active surface areas of the electrodes and/or the interelectrode spacings in bipolar leads have been recognized to be of significance for various reasons, including far-field signal rejection or attenuation.
For example, U.S. Pat. No. 5,899,929 relates to a bipolar lead system for inducing ventricular tachycardia utilizing near-field T-wave sensing to determine the optimum parameters for anti-tachycardia stimulation, with the goal of accurately sensing the T-wave. U.S. Pat. No. 5,342,414 discloses a bipolar defibrillation lead for placement in the right ventricle. The lead is designed to place the defibrillation electrode as close to the apex of the right ventricle as practicable while retaining an adequately spaced bipolar (helical tip and ring) electrode pair for sensing the ventricular depolarizations. Thus, the ring electrode is located at the most distal extremity of the lead body permitting the proximal defibrillation electrode to be positioned correspondingly close to the lead's distal extremity. An interelectrode spacing of 5 mm between the distal end of the ring electrode and the proximal end of the active portion of the helical tip electrode is disclosed for adequate ventricular depolarization sensing. Electrode areas are not described and far-field signal rejection is not dealt with, let alone in the context of atrial pacing and sensing.
U.S. Patent Application Publication US2002/0123784A1 discloses a tri-polar pacing and sensing lead for use with an implantable medical device. The tri-polar lead includes three electrodes separated from each other to maximize sensing and pacing activities. A first ring electrode is located on the tri-polar lead within about 1.0 mm from a tip electrode. A second ring electrode is positioned on the tri-polar lead in the range of about 10.0 to 30.0 mm from the tip electrode. The electrode pair comprising the tip and first ring electrodes provides local sensing capabilities within either the atrium or the ventricle, while the electrode pair comprising the tip and second ring electrodes provides pacing capabilities. It will be seen that this tri-polar arrangement, although said to be capable of attenuating far-field pacing artifacts such as R-wave spikes which can contaminate sensing by an atrial lead, requires, in contrast to a bipolar lead, two pairs of electrodes to provide pacing and sensing and accordingly, requires a third electrical conductor to connect the additional ring electrode to the implantable medical device.
As illustrated by U.S. Pat. No. 5,476,496, the disclosure of which is hereby expressly incorporated by reference, it is known that in a bipolar pacing and sensing lead, the indifferent electrode (or anode), typically in the form of an electrically conductive ring disposed proximally of the tip cathode electrode, should have a large active surface area compared to that of the cathode. The objects of such an areal relationship are to reduce the current density in the region surrounding the anode so as to prevent needless or unwanted stimulation of body tissue around the anode when a stimulation pulse is generated between the cathode and anode, and to minimize creation of two focal pacing sites, one at the cathode and one at the anode which could promote arrhythmia. Typically, the total surface area of the anode is selected so as to be about two times to about six times that of the cathode.
Despite the advances in the field, there remains a need for a bipolar pacing and sensing lead with electrode parameters optimized to sufficiently attenuate far-field R-wave signals while at the same time providing clinically acceptable near-field P-wave signals for reliable sensing. Moreover, the need exists for such a lead that can be located in association with either the right ventricle or left ventricle, and that can sufficiently attenuate T-waves and far-field P-wave signals.